Whenever a car is to be parked into a space with a limited width and length, such as a garage, or a space on a road side between two parked cars, the car has to be moved skillfully. Usually, a driver has to use a visual measuring method to judge if the parking space is wide and long enough, and to drive the car backwards skillfully. In case of the visual measurement being incorrect or the driving skill being poor, the parking operation encounters difficulty or the car has to be driven back and forth repeatedly, wasting a lot of time and possibly scratching the car body with other cars or objects.